1. Technical Field
The present disclosure is generally related to communication systems and methods and, more particularly, is related to collision avoidance systems and methods in wireless networks.
2. Related Art
Communication networks come in a variety of forms. Notable networks include wireline and wireless. Wireline networks include local area networks (LANs), DSL networks, and cable networks, among others. Wireless networks include cellular telephone networks, classic land mobile radio networks and satellite transmission networks, among others. These wireless networks are typically characterized as wide area networks. More recently, wireless local area networks and wireless home networks have been proposed, and standards, such as Bluetooth and IEEE 802.11, have been introduced to govern the development of wireless equipment for such localized networks.
A wireless local area network (LAN) typically uses infrared (IR) or radio frequency (RF) communication channels to communicate between portable or mobile computer terminals and access points (APs) or base stations. These APs are, in turn, connected by a wired or wireless communications channel to a network infrastructure which connects groups of access points together to form the LAN, including, optionally, one or more host computer systems.
Wireless protocols such as Bluetooth and IEEE 802.11 support the logical interconnections of such portable roaming terminals having a variety of types of communication capabilities to host computers. The logical interconnections are based upon an infrastructure in which at least some of the terminals are capable of communicating with at least two of the APs when located within a predetermined range, each terminal being normally associated, and in communication, with a single one of the access points. Based on the overall spatial layout, response time, and loading requirements of the network, different networking schemes and communication protocols have been designed so as to most efficiently regulate the communications.
IEEE Standard 802.11 (“802.11”) is set out in “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications” and is available from the IEEE Standards Department, Piscataway, N.J. The IEEE 802.11 standard permits either IR or RF communications at 1 Mbps, 2 Mbps and higher data rates, a medium access technique similar to carrier sense multiple access/collision avoidance (CSMA/CA), a power-save mode for battery-operated mobile stations, seamless roaming in a full cellular network, high throughput operation, diverse antenna systems designed to eliminate “dead spots,” and an easy interface to existing network infrastructures. The IEEE Standard 802.11b extension supports data rates up to 11 Mbps.
The current 802.11 standard describes several methods to set a virtual carrier sense, referred to as a network allocation vector or NAV, most notably by request to send/clear to send (RTS/CTS) mechanisms. The sending of an RTS frame (herein, RTS frame also referred to as, simply, RTS) sets a NAV locally around the sender of the RTS, and the sending of a CTS frame (herein, CTS frame is also referred to as CTS) does the same locally around the sender of the CTS (e.g., the receiver of the RTS).
One problem that exists in current implementations under 802.11 involves what is referred to as a hidden node problem. For instance, in an infrastructure mode of a wireless LAN system, a first device may communicate frames to the AP and vice versa. Similarly, a second device may communicate frames to the AP, and vice versa. However, the second device may not detect transmissions from the first device (hence the phrase hidden node), for instance if the distance between the first and second device is too great. Because of the hidden node problem, various complications may arise in terms of collision avoidance and symmetry of response among devices, causing inequity in terms of opportunities for access to a shared medium of the communication system. For instance, in 802.11 compliant systems, the NAV can be reset by an access point (AP) through the transmission of what is commonly referred to as a CF-end frame. One exemplary frame format for a CF-end frame 10 is shown in FIG. 1, and comprises two octets (exemplary quantity of octets shown beneath the frame 10) corresponding to a frame control field 102, two octets corresponding to duration field 104, six octets corresponding to a receiver address (RA) field 106, which usually contains the Broadcast Address (BC), six octets corresponding to a basic service set identifier (BSSID) field 108, and four octets corresponding to a frame check sequence (FCS) field 110. When the distance between a client station and an AP is large, one possible effect is that the area where the CF-end frame 10 is not received will not reset the NAV, resulting in inequitable access as described above.